Energy Efficiency of a transformer is defined as the ratio of total energy output for a certain period to the total energy input for the same period.

All Day Efficiency is defined as the ratio of the energy output to the energy input taken over a 24 hour period. All Day Efficiency is basically Energy Efficiency of Transformer calculated for a period of 24 hours. Mathematically, we can write,


What is the need of All Day Efficiency?

To understand the need for calculating All Day Efficiency, first, we will discuss the basic difference between Power Transformer and Distribution Transformer.

Power Transformers are used for sending and receiving ends of a long, high voltage transmission line to step up or step down the voltage level. These Transformers are manipulated to operate near their rated capacity. Therefore, Power Transformers are disconnected during light load periods. In view of this, a Power Transformer is designed to have maximum efficiency at or near rated capacity. Therefore, the choice of Power Transformer out of a large number of competing Transformers shall be based on full load efficiency.

Now, Distribution Transformers are those which changes the voltage level to a level suitable for utilization purpose at the consumer’s premises. A Power Transformer doesn’t come in direct contact with the consumer’s terminal whereas a Distribution Transformer must have its secondary directly connected to consumer’s terminals. The load on a Distribution Transformer varies a wide range of 24 hours a day. For example, a Distribution Transformer may have practically no load or little load during a considerable period of the day but in evening time it may go up to the rated capacity. In this way, the primary of Distribution Transformer is always energized irrespective of load. Notice here the difference between the Power and Distribution Transformer. As the primary of Distribution Transformer is always energized, core loss will take place continuously. In view of this, Distribution Transformers are designed to have a very low value of core loss.

But we know that,

kVA Load for maximum efficiency = Rated Load x n


Pc = Core Loss

Psc = Full Load ohmic loss

Thus from the above equation, for the reduced core loss Pc, the maximum efficiency may occur at about one half of its rated kVA. Thus Distribution Transformer should not be judged by its full load efficiency which is usually much less than its maximum efficiency. However, the choice of Distribution Transformer out of a large number of competing transformers shall be based on All Day Efficiency. This is the essence of the term All Day Efficiency.

All Day Efficiency of Distribution Transformer

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